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various mixtures m fo or periods of time ranging r fro om hours tto months, and theirr surfaces treated t and d retreated to try and d match th he appearannce of the weathered d surface off the kouro os. Such tessts yielded only a few w examples that appeaared similarr to the su urface of kouros. k Evven those samples, however, were diffeerent when n examined d under hig gh magnificcation or subjected s to o geochem mical analyssis. In factt, all of the samples clearly show wed that th hey were th he result off recent alteration and d not long-tterm weath hering proccesses. While thee scientificc tests hav e been unaable to unequivocablly prove au uthenticity y, they havee shown th hat the weaathered surface layer of the kouuros bears more simi-larities to o naturally y occurrinng weatherred surfac ces than tto known artificially y produced surfaces. Furthermoore, there is i no evide ence indicaating that the t surfacee alteration of the kouros is of modern origin. In spite of intensive study by scientists, archaeologists, and art historians, opinion is still divided as to the authenticity of the Getty kouros. Most scientists accept that the kouros was carved sometime around 530 B.C., but most art historians are doubtful. Pointing to inconsistencies in its style of sculpture for that period, they argue that it is a modern forgery. Regardless of the ultimate conclusion on the Getty kouros, geological testing to authenticate marble sculptures is now an important part of many museums' curatorial functions. In addition, a large body of data about the characteristics and origin of marble is being amassed as more sculptures and quarries are analyzed. INTRODUCTION Metamorphic rocks (from the Greek meta meaning change and I/1O'1I/Ill meaning shape) are the third major group of rocks. They result from the transformation of other rocks by metamorphic processes that usually occur beneath the Earth's surface (see Figure 1-11). During metamorphism, rocks are subjected to sufficient heat, pressure, and fluid activity to change their mineral composition and/or texture, thus forming new rocks. These transformations take place in the solid state, and the type of metamorphic rock formed depends on the original composition and texture of the parent rock the agents of metamorphism, and the amount of time the parent rock was subjected to the effects of metamorphism. A large portion of the Earth's continental crust is composed of metamorphic and igneous rocks. Together, they form the crystalline basement rocks that underlie the sedimentary rocks of a continent's surface. This basement rock is widely exposed in regions of the continents known as shields, which have been very stable during the past 600 million years (~ Figure 8-2). Metamorphic rocks also constitute a sizable portion of the crystalline core of large mountain ranges some of the oldest known rocks, dated at 3.96 billion years from the Canadian Shield, are metamorphic, indicating they formed from even older rocks. Why is it important to study metamorphic rocks? For one thing they provide information about geological processes operating within the Earth and about the way these processes have varied through time. Furthermore, metamorphic rocks such as marble and slate are used as building materials, and certain metamorphic minerals are economically important. For example, talc is used in cosmetics, in the manufacture of paint, and as a lubricant, while asbestos is used for insulation and fireproofing (see Perspective 8-1). THE AGENTS OF METAMORPHISM The three agents of metamorphism ate heat, pressure, and fluid activity. During metamorphism, the original rock undergoes change so as to achieve equilibrium with its new environment. The changes may result in the formation of new minerals and/or a change in the texture of the rock by the reorientation of the original minerals. In some instances the change is minor, and features of the parent rock can still be recognized. In other cases the rock changes so much that the identity of the parent rock can be determined only with great difficulty, if at all. In addition to heat, pressure, and fluid activity, time is also important to the metamorphic process. Chemical reactions proceed at different rates and thus require different amounts of time to complete. Reactions involving silicate compounds are particularly slow, and because most metamorphic rocks are composed of silicate minerals, it is thought that metamorphism is a slow geologic process. Heat Heat is an important agent of metamorphism because it increases the rate of chemical reactions that may produce minerals different from those in the original rock. The heat may come from intrusive magmas or result from deep burial in the Earth's crust such as occurs during subduction along a convergent plate boundary. When rocks are intruded by bodies of magma, they are subjected to intense heat that affects the surrounding rock; the most intense heating usually occurs adjacent to the magma body and gradually decreases with distance from the intrusion. The zone of metamorphosed rocks that forms in the country rock adjacent to an intrusive igneous body is usually rather distinct and easy to recognize. Pressure When rocks are buried, they are subjected to increasingly greater lithostatic pressure; this pressure, which results from the weight of the overlying rocks, is applied equally in all directions (Figure 8-3). As rocks are subjected to increasing lithostatic pressure with depth, the mineral grains within a rock may become more closely packed. Under such conditions, the minerals may recrystallize; that is, they may form smaller and denser minerals. In addition to the lithostatic pressure resulting from burial, rocks may also experience differential pressures (~ Figure 11-4). In this case, the pressures are not equal on all sides and the rock is consequently distorted. Differential pressures typically occur during deformation associated with mountain building and can produce distinctive metamorphic textures and features. Fluid Activity In almost every region where metamorphism occurs, water and carbon dioxide (CO2) are present in varying amounts along mineral grain boundaries or in the pore spaces of rocks. These fluids which may contain ions in solution enhance metamorphism by increasing the rate of chemical reactions. Under dry conditions, most minerals react very slowly, but when even small amounts of fluid are introduced, reaction rates increase, mainly because ions can move readily through the fluid and thus enhance chemical reactions and the formation of new minerals. The following reaction provides a good example of how new minerals can be formed by fluid activity. Here, seawater moving through hot basaltic rock of the oceanic crust transforms olivine into the metamorphic mineral serpentine: 2Mg2SiO4 + 2H2O Mg3Si2O5(OH)4 + MgO olivine water serpentine carried away in solution The cheemically acttive fluids thhat are partt of the mettamorphic pprocess com me primarilyy from threee sources. The T first is w water trappeed in the po ore spaces oof sedimentaary rocks ass they form m. A second d is the voolatile fluid d within mab'l11a. m Thhe third so ource is thee dehydratioon of water--bearing miinerals such h as gypsum (CaSO4.2H H20) and some clays. TYPES OF O METAM MORPHISM M Three maajor types of metamoorphism aree recognizeed: contact m metamorphism m in whichh magmatic heat and fluids act to producce change; dynamic m metamorphism m, which iss principallyy the result of high diffferential prressures asssociated witth intense deformation d n; and regionnal metamorpphism, whichh occurs within a large area and is caused primarily p byy mountain--building fo orces. Evenn though we w will disccuss each ttype of metamorphism m separatelyy, the bound dary betweeen them is not alwayss distinct an and dependss largely onn which of the t three meetamorphic agents was dominant. Contact Metamorph M hism Contact metamorphi m n a body of o magma aalters the surrounding s g sm takes pplace when country roock. At shallow depthhs an intru uding magm ma raises th the temperaature of thee surroundinng rock, cau using therm mal alteratio on. Furtherm more, the reelease of hot fluids intoo the countrry rock by th he cooling iintrusion caan also aid in n the formaation of new w minerals. Importannt factors in n contact m metamorphissm are the initial i tempperature and d size of thee intrusion as well as the fluid ccontent of the t magma and/or couuntry rock. The initiaal temperatuure of an inttrusion is coontrolled, in n part, by itts compositiion: mafic magmas m aree hotter thann felsic mag gmas (see C Chapter 4) and a hence haave a greateer thermal effect e on thee rocks direectly surroun nding them m. The size of o the intrusion is alsoo important. In the casee of small inntrusions, su uch as dikess and sills, usually u only y those rockks in immed diate contact with the inntrusion aree affected. B Because larrge intrusion ns, such as batholiths, take a longg time to coool, the increeased tempeerature in th he surround ding rock maay last long g enough forr a larger arrea to be afffected. Fluids also a play an n important role in con ntact metam morphism. M Many magm mas are wet and contain hot, chem mically acttive fluids that t may em manate intoo the surrou unding rockk. These fluids can react with the rock and aid in the formation of new minerals. In addition, the country rock may contain pore fluids that, when heated by the magma, also increase reaction rates. Temperatures can reach nearly 900°C adjacent to an intrusion, but they gradually decrease with distance. The effects of such heat and the resulting chemical reactions usually occur in concentric zones known as aureoles (~ Figure 8-5). The boundary between an intrusion and its aureole may be either sharp or transitional (~ Figure 8-6). Metamorphic aureoles vary in width depending on the size, temperature, and composition of the intrusion as well as the composition of the surrounding country rock. Typically, large intrusive bodies have several metamorphic zones, each characterized by distinctive mineral assemblages indicating the decrease in temperature with distance from the intrusion (Figure 8-5). The zone closest to the intrusion, and hence subject to the highest temperatures, may contain high temperature metamorphic minerals (that is, minerals in equilibrium with the higher temperature environment) such as sillimanite. The outer zones may be characterized by lower-temperature metamorphic minerals such as chlorite, talc, and epidote. The formation of new minerals by contact metamorphism depends not only on proximity to the intrusion, but also on the composition of the country rock. Shales, mudstones, impure limestones, and impure dolostones are particularly susceptible to the formation of new minerals by contact metamorphism, whereas pure sandstones or pure limestones typically are not. Two types of contact metamorphic rocks are generally recognized: those resulting from baking of country rock and those altered by hot solutions. Many of the rocks resulting from contact metamorphism have the texture of porcelain; that is, they are hard and fine grained. This is particularly true for rocks with a high clay content, such as shale. Such texture results because the clay minerals in the rock are baked, just as a clay pot is baked when fired in a kiln. During the final stages of cooling when an intruding magma begins to crystallize, large amounts of hot, watery solutions are often released. These solutions may react with the country rock and produce new metamorphic minerals. This process, which usually occurs near the Earth's surface, is called hydrothermal alteration, and may result in valuable mineral deposits. Geologists think that many of the world's ore deposits result from the migration of metallic ions in hydrothermal solutions. Examples include copper, gold, iron ores, tin, and zinc in various localities including Australia, Canada, China, Cyprus, Finland, Russia, and the western United States. Dynamic Metamorphism Most dynamic metamorphism is associated with fault (fractures along which movement has occurred) zones where rocks are subjected to high differential pressures. The metamorphic rocks resulting from pure dynamic metamorphism are called Mylonites, and they are typically restricted to narrow zones adjacent to faults. Mylonites are hard, dense, fine-grained rocks, many of which are characterized by thin laminations. Tectonic settings where mylonites occur include the Moine Thrust Zone in northwest Scotland and portions of the San Andreas fault in California. Regional Metamorphism Most metamorphic rocks result from regional metamorphism, which occurs over a large area and is usually caused by tremendous temperatures, pressures, and deformation within the deeper portions of the Earth's crust. Regional metamorphism is most obvious along convergent plate margins where rocks are intensely deformed and recrystallized during convergence and subduction. Within these metamorphic rocks, there is usually a gradation of metamorphic intensity from areas that were subjected to the most intense pressures and/or highest temperatures to areas of lower pressures and temperatures. Such a gradation in metamorphism can be recognized by the metamorphic minerals that are present. Regional metamorphism is not just confined to convergent margins. It also occurs in areas where plates diverge, though usually at much shallower depths because of the high geothermal gradient associated with these areas. From field studies and laboratory experiments, certain minerals are known to form only within specific temperature and pressure ranges. Such minerals are known as index minerals because their presence allows geologists to recognize low-, intermediate-, and high-grade metamorphic zones ) Table 8-1). A typical progression of index minerals forming primarily in rocks that were originally clay rich involves the sequential formation of the following minerals: chlorite ~ biotite ~ amphibole ~ staurolite ~ sillimanite. Different rock compositions, though, develop different index minerals. When sandy dolomites are metamorphosed they produce an entirely different set of index minerals. Therefore, a specific set of index minerals commonly forms in particular rock types as metamorphism progresses. CLASSIFICATION OF METAMORPHIC ROCKS For purposes of classification, metamorphic rocks are commonly divided into two groups: those exhibiting a foliated texture and those with a nonfoliated texture (@) Table 82). Foliated Metamorphic Rocks Rocks subjected to heat and differential pressure during metamorphism typically have minerals arranged in a parallel fashion that gives them a foliated texture (~ Figure 8-7). The size and shape of the mineral grains determine whether the foliation is fine or coarse. If the foliation is such that the individual grains cannot be recognized without magnification, the rock is said to be slate (~ Figure 8-8). A coarse foliation results when granular minerals such as quartz and feldspar are segregated into roughly parallel and streaky zones that differ in composition and color as in a gneiss (~ Figure 8-10). Foliated metamorphic rocks can be arranged in order of increasingly coarse grain size and perfection of foliation. Slate is a very fine-grained metamorphic rock that commonly exhibits slaty cleavage (Figure 8-8b). Slate is the result of low-grade regional metamorphism of shale or, more rarely, volcanic ash. Because it can easily be split along cleavage planes into A at pieces, slate is an excellent rock for roofing and floor tiles, billiard and pool table tops, and blackboards. The different colors of most slates are caused by minute amounts of graphite (black), iron oxide (red and purple), and/or chlorite (green). Phyllite is similar in composition to slate, but is coarser grained. The minerals, however, are still too small to be identified without magnification. Phyllite can be distinguished from slate by its glossy or lustrous sheen. It represents an intermediate grain size between slate and schist. Schist is most commonly produced by regional metamorphism. The type of schist formed depends on the intensity of metamorphism and the character of the parent rock (~ Figure 8-9). Metamorphism of many rock types can yield schist, but most schist appears to have formed from clay-rich sedimentary rocks (Table 8-2). All schists contain more than 50% platy and elongated minerals, all of which are large enough to be clearly visible. Their mineral composition imparts a schistosity or schistose foliation to the rock that usually produces a wavy type of parting when split. Schistosity is common in low- to high grade metamorphic environments, and each type of schist is known by its most conspicuous mineral or minerals, such as mica schist. chlorite schist, or talc schist. Gneiss is a metamorphic rock that is streaked or has segregated bands of light and dark minerals. Gneisses are composed mostly of granular minerals such as quartz and/or feldspar with lesser percentages of platy or elongated minerals such as micas or amphiboles (Figure 8-10). Quartz and feldspar are the principal light-colored minerals, while biotite and hornblende are the typical dark-colored minerals. Most gneiss breaks in an irregular manner, much like coarsely crystalline nonfoliated rocks. Most gneiss probably results from recrystallization of clay-rich sedimentary rocks during regional metamorphism (Table 8-2). Gneiss also can form from igneous rocks such as granite or older metamorphic rocks. Another f.1irly common foliated metamorphic rock is "amphibolite. It is a dark-colored rock and composed mainly of hornblende and plagioclase. The alignment of the horn- blende crystals produces a slightly foliated texture. Many amphibolite’s result from medium- to high-grade metamorphism of such ferromagnesian mineral-rich igneous rocks as basalt. In some areas of regional metamorphism, exposures of "mixed rocks" having both igneous and high-grade metamorphic characteristics are present. In these rocks, called migmatites, streaks or lenses of granite are usually intermixed with high-grade ferromagnesian-rich metamorphic rocks, imparting a wavy appearance to the rock (~ figure 8-11). Most migmatites are thought to be the product of extremely high-grade metamorphism, and several models for their origin have been proposed. Part of the problem in determining the origin of migmatites is explaining how the granitic component formed. According to one model, the granitic magma formed in place by the partial melting of rock during intense metamorphism. Such an origin is possible providing that the host rocks contained quartz and feldspars and that water was present. Another possibility is that the granitic components formed by the redistribution of minerals by recrystallization in the solid state, that is, by pure metamorphism. Nonfoliated Metamorphic Rocks In some metamorphic rocks, the mineral grains do not show a discernible preferred orientation. Instead, these rocks consist of a mosaic of roughly equidimensional minerals and are characterized as having a nonfoliated texture (~ Figure 8-12). Most nonfoliated metamorphic rocks result from contact or regional metamorphism of rocks in which no platy or elongate minerals are present. Frequently, the only indication that a granular rock has been metamorphosed is the large grain size resulting from recrystallization. Nonfoliated metamorphic rocks are generally of two types: those composed mainly of only one mineral, for example, marble or quartzite; and those in which the different mineral grains are too small to be seen without magnification, such as greenstone and hornfels. Marble is a well-known metamorphic rock composed predominantly of calcite or dolomite; its grain size ranges from fine to coarsely granular (~ Figure 8-13). Marble results from either contact or regional metamorphism of limestones or dolostones (Table 8-2). Pure marble is snowy white or bluish, but varieties of all colors exist because of the presence of mineral impurities in the parent sedimentary rock. The softness of marble, its uniform texture, and its various colors have made it the favorite rock of builders and sculptors throughout history (see the Prologue). Quartzite is a hard, compact rock formed from quartz sandstone under medium-tohigh-grade metamorphic conditions during contact or regional metamorphism (~ Figure 8-14). Because recrystallization is so complete, metamorphic quartzite is of uniform strength and therefore usually breaks across the component quartz grains rather than around them when it is struck. Pure quartzite is white, but iron and other impurities commonly impart reddish or other color to it. Quartzite is commonly used as foundation material for road and railway beds. The name greens/one is applied to any compact, dark green, altered, mafic igneous rock that formed under low to-high-grade metamorphic conditions. The green color results from the presence of chlorite, epidote, and hornblende. Hornfels is a fine-grained, nonfoliated metamorphic rock resulting from contact metamorphism; it is composed of various equidimensional mineral grains. The composition of hornfels is directly dependent upon the composition of the parent rock, and many compositional varieties are known. The majority of hornfels, however, are apparently derived from contact metamorphism of clay-rich sedimentary rocks or impure dolostones. Anthracite is a black, lustrous, hard coal that contains a high percentage of fixed carbon and a low percentage of other elements. It usually forms from the metamorphism of various types of coals by heat and pressure and is thus considered by many geologists to be a metamorphic rock. METAMO ORPHISM M AND PLA ATE TECT TONICS hism is asssociated wiith all threee types of plate boun ndaries (seee Although metamorph m commoon along co onvergent plate p marginns. Metamo orphic rockss Figure 1-110), it is most form at convergent plate p bounddaries becaause temperrature and ppressure in ncrease as a result of plate p collisio ons. Very slow wly, and mettamorphism m is caused mostly by increasing i ppressure witth depth. Ass subductionn continuess, both tempperature and d pressure increase i witth depth an nd can result in high-grrade metam morphic rockks. Eventuaally, the desscending pllate begins to melt andd generates a magma that t moves upward. This rising magma m mayy alter the surroundingg rock by coontact metam morphism, producing migmatites m in the deepper portions of the crust and hornffels at shaallower deppths. Such an environment is ccharacterizeed by highh temperatuures and low w to medium m pressures. While metamorph m ism is moost commo on along convergent c plate margins, manyy divergent plate bound daries are chharacterized d by contactt metamorphhism. Risin ng magma at midoceaniic ridges heeats the adj djacent rock ks, producin ng contact metamorph hic mineralss and texturres. In addition to conntact metam morphism, fluids emannating from m the risingg magma annd from the reaction of the mag gma and seea water-ver ery common nly producee hydrotherm mal solution ns that mayy precipitate minerals of economic value. MORPHIS SM AND NA ATURAL RESOURC R CE METAM Many metamorphic m c rocks are valuable natural n resou urce. Whilee these include variouss types of ore o depositss, the two most familliar and wid dely used m metamorphiic rocks, ass such, are marble m and slate, whicch, as previo ously discusssed, have bbeen used for f centuriess in a varietty of ways. Many orre deposits result from contact meetamorphism m during whhich hot, ion n-rich fluidss migrate frrom igneous intrusionss into the surrounding rock, thereeby produciing rich oree deposits. The T most co ommon sulffide ore min nerals assocciated with contact metamorphism m are bornitte, chalcopy yrite, galenaa, pyrite, an nd sphalerite, while tw wo common n oxide oree minerals are a hematitee and magneetite. Tin an nd tungsten are also im mportant ores associatedd with contaact metamorrphism (Tabble 8-3). Other ecconomically y importantt metamorphic mineralls include taalc for talcu um powderr; graphite for fo pencils and a dry lubbricants; garrnets and co orundum, w which are ussed as abrasives or gemstones, depending d oon their quaality; and an ndalusite, ky kyanite, and sillimanitee, nufacture off high-tempeerature porccelains and temperaturee all of whicch are used in the manu resistant minerals m for products suuch as spark kplugs and the t linings oof furnaces.. Ansswers Additiona al Readingss